Systems and methods to extinguish fires in underground coal mine passages providing a closed volume seal in the passage have been developed; descriptions of exemplary systems and methods are found in U.S. Pat. Nos. 3,469,405 (Reinhold), 3,583,165 (West), and 3,500,934 (Magnuson). In certain of the prior art systems and methods, the closed volume seal is provided by boring into the passage and forcing sealant material into the passage through the bore. The sealant fills a cross-sectional area of the passage, thereby preventing the flow of gas through the passage from one side of the seal to the other, to enable the fire to be contained and thereafter extinguished. The technique can also be utilized to seal off mine passages that are no longer being utilized so that if a fire starts in one portion of a mine, it will not spread to another portion thereof.
It is desired to determine the effectiveness of such seals, preferably with measuring equipment that can be read at a site remote from the seal and the passage containing the seal. Also, apparatus for determining the effectiveness of the seal should:
1. INDICATE THE PERCENTAGE COMPLETION OF THE SEAL AS THE SEAL IS EMPLACED;
2. PROVIDE A DEFINITE INDICATION OF A SEAL HAVING BEEN COMPLETED; AND
3. INDICATE THE SIZE OF AN UNSEALABLE AREA IF A COMPLETE SEAL IS NOT FEASIBLE. It is necessary to determine these factors to establish the need for additional seal material, the need for additional seals in the mine, and the likely risk of one entering the mine.
The cross-sectional areas of passages to be blocked by the seal typically range from less than 50 sq. ft. to in excess of 150 sq. ft. Since leakage is most likely to occur at the seal perimeter, which may range from 30 ft. to nearly 60 ft., the likely leakage region also has a relatively wide range. The sealing barrier is relatively thick, typically being 15 ft. thick at roof level and 100 ft. on the floor of the passage. Because of these wide variations in seal geometry, as well as different gas flow patterns in the passage on either side of the seal, the pressure differential across the seal is susceptible to wide variations both in magnitude and direction, thereby complicating monitoring techniques relying on fluid mass transfer across the seal in a preferred direction.
Flanking paths along one or more passages parallel to the passage being sealed, as well as through cracks in the rock and coal of the mine, short circuit the seal being constructed after the seal has finally been emplaced. It is important to insure the completeness of each seal immediately after it has been formed, even in the presence of the flanking paths, to eliminate the expense and time consumed in returning to bore holes to top off a seal, i.e., to finally complete a seal. To permit checking of individual seals, the seal monitoring system must distinguish between paths through the seal and the flanking paths around the seal.
Several different methods and/or apparatus are known for determining the effectiveness of a seal. Some of the better known seal monitoring techniques and apparatus involve:
1. adding trace elements on one side of a seal and monitoring the elements which reach the other side;
2. monitoring the increase or decrease in fluid volume on one side of the seal;
3. coating one side of the seal with an impervious, flexible layer and searching for bubbles in the layer; and
4. detecting acoustic energy of fluid escaping through the seal, by utilizing passive acoustic techniques. Of these four approaches, all of which have disadvantages relating to at least one of the previously enumerated areas, only the last is pertinent to the present invention.
The passive acoustic technique relies on the physical mechanism of air or another gas escaping through an orifice producing sonic energy primarily in the 30 KHz to 50 KHz band. Ultrasonic equipment is available that relies on this mechanism. The equipment is utilized, for example, to detect leaks in telephone cables, tires, ducts, and other pressurized systems. From our investigations, it does not appear that the ultrasonic equipment for detecting gas escaping through an orifice is suitable for detecting flow across seals in a mine passage. In particular, the pressure drop across and flow rate through a mine passage seal are typically so low as to be undetectable by a transducer positioned in proximity to the seal-passage interface.
While passive acoustic devices responsive to much lower frequencies are used to detect the flow of water in soil, the presence of background noise at these frequencies makes the devices unsuitable for the present problem. In particular, the flow of water against the seal in passageways close to the seal and in strata of the earth close to the seal is in the same frequency range as gas that might leak through the seal to cause a likely masking of the leaking gas.